The present invention relates to filters and it will be explained with reference to a particular application which is speech processing for converting it into digital signals before transmission over a telephone line. This very important application shows very well the interest of the filter of the invention but other applications may of course be considered for this filter.
In digital telephone transmission, the speech frequency signals from a microphone undergo filtering so as to retain only signals in a desired frequency band (200 to 3400 Hertz for example) before being applied to an analog-digital converter which operates in a sampled fashion at a frequency for example of 8 kHz (much higher than the speech frequencies transmitted). The analog-digital converter examines then every 125 microseconds the level of the filtered speech signal and converts it into a binary digital value which is fed to the telephone line.
For reasons of ease of integration on very small-sized semiconductor wafers, the filtering is essentially provided by means of switched capacity filters (according to a principle described more especially in the articles of the review IEEE Journal of Solid State Circuits, vol SC-12, no. 6, December 1977, pages 592-608). In these filters, the resistances of the circuits are replaced by capacitors switched at a switching frequency higher than the frequencies to be transmitted through the filter. For example, the switching frequency may be 128 kHz.
The advantage of switched capacity filters is that they allow very low cut-off frequencies to be obtained (e.g. a high-pass filter cutting off at 200 Hz) without requiring components which would take up too much room on a silicon surface in an integrated circuit. In the application under consideration, they allow more especially filters and analog-digital coder to be placed on the same substrate, and a filtering and coding circuit to be formed on the same substrate for transmitting and a decoding and filtering circuit for reception (codec+filter).
FIG. 1 shows a possible filtering and coding arrangement with a first high-pass filter 10, for example of the third order, with a cut-off frequency of about 200 Hz, then a low-pass filter 12, for example of the fifth order with a cut-off frequency of about 3.4 kHz, then an analog-digital converter 14.
Unfortunately, the low-pass filter which is an active filter formed from operational amplifiers, presents at its output a not inconsiderable offset voltage (for example of the order of 200 millivolts) which is interpreted by the converter as a signal level to be coded digitally. This is particularly troublesome because the coding law of the converter is established with high compression for improvement of the dynamic extent of the transmitted signals. Thus, the weak signals are identified with coding levels much closer together than the stronger signals. The variations of the offset voltage at the output shift the overall level of the weak signals and give them an apparent digital value all the more erroneous since the dynamic compression is more pronounced.
One means for eliminating the residual output offset due to the amplifiers would be to insert and RC network with a capacitor 16 in series between the low-pass filter 12 and coder 14 (FIG. 2). But, considering the frequencies to be transmitted, the capacitor 16 must be too large (of the order of 40 nanofarads) for integration thereof on the same substrate as the filter and the coder. It must then be provided externally, with two terminals for access to the integrated circuit, and it is known that the number of external access terminals of an integrated circuit must be reduced as much as possible.
So provision has been proposed (FIG. 3) of a control loop acting on a source 18 for compensating the residual offset, this loop taking as input signal the sign bit of the converter 14 and acting on the compensation source so that on average the sign bit is as often at 0 as at 1; in fact, over an average period of time, the speech signal must present a zero mean value. If a sign bit is more often at 1 than at 0, it is because there is positive residual offset to be compensated for. But the control loop thus formed has a not inconsiderable time constant which affects the response time of the coder. Moreover, it itself requires a filtering capacitor 20, with however a single external access terminal in the integrated circuit (FIG. 3).
To avoid this high capacity and the external access terminal(s), a different solution is proposed here by starting from the idea that the high-pass filter, which could as has been said be of the third order, is broken down into an upstream high-pass filter, for example of the second order, and a high-pass filter of the first order which presents no residual offset voltage and which is placed downstream of the low-pass filter, immediately upstream of the converter.